Thus, the exact question should be more alike this one : what address should I choose, in the Oric memory, as a temporary variable ???
I imagine that it is not well indicated to choose a zero page address, or am I wrong ?

;A<-A+X
CLC
CLD
LDA #$03
LDX #$02
PHA ;push A on the stack
TXA ;move X to A
TSX ;move address of the stack in X
ADC $00,X ;use X as index in order to add what is contained on the stack to A
;(hum, is this allowed ???? lol )
;what follows does not really matters to me
STA result ;result is a memory location
PLA ;Pop A, let's be clean

under basic, zero page addresses 0 to B, BB-BC and F3 to F9 are not used and may be used by you (the programmer).
if a program is totally machine code and doesn't use basic, then you get all 256 addresses of zero page, page 2, etc.